The invention relates to methods and apparatus for making a heat-insulating paper container having a foamed layer on at least one surface thereof. More particularly, the invention relates to methods and apparatus for conveying a container through an oven to produce a foamed layer.
Disclosed in U.S. Pat. Nos. 4,435,344 and 5,490,631 (the disclosures of which are incorporated by reference herein in their entirety) is a heat-insulating paper container comprising a side body and a bottom panel member, characterized in that at least one surface of the side body is coated or laminated with a foamed heat-insulating layer of a thermoplastic synthetic resin film.
FIG. 5 is a simplified cross section of an example of such a prior art heat-insulating paper container 1. The container 1 comprises basically a paper sheet forming a side body 3 and a bottom panel member 5. The side body 3 has a foamed heat-insulating layer 11 which entirely covers its outer surface and which may typically be formed of a thermoplastic synthetic resin such as polyethylene.
The inner surface of the side body 3 is covered with a film 13 which is made of either a thermoplastic synthetic resin such as polyethylene, or an aluminum foil. The inner surface of the bottom panel member 5 is laminated with a thermoplastic synthetic resin film 15.
The heat-insulating container 1 is fabricated by means of a cup-forming machine (not shown). First, the outer surface of a paper sheet is extrusion-laminated with a film of thermoplastic synthetic resin such as polyethylene. The inner surface of the paper sheet is also laminated with a thermoplastic synthetic resin film or an aluminum foil. A paper sheet for making the bottom panel member is laminated with a thermoplastic synthetic resin film on one side only. A blank is cut from each of the paper sheets. Using a conventional cup-forming machine, the two blanks are fabricated into a container, with the blank or the bottom panel member oriented in such a way that the film laminated side faces inward. The thus-fabricated untreated container is then subjected to a heat treatment in order to cause moisture in the paper to vaporize. For instance, the untreated containers can be heated at 120.degree. C. for 120 seconds.
According to U.S. Pat. No. 4,435,344 (column 4, lines 5-11), the containers may be heat-treated by being conveyed through an oven. The conveyance of the containers through the oven may be performed by laying the containers en masse onto a conveyor belt, the containers could be disposed right side up, but in order to achieve maximum cup stability, the cups would preferably be in an inverted state, i.e., supported on their larger-diameter rims. Nevertheless, as the containers pass through the oven, they would be subjected to air currents and conveyor vibrations which would cause the very light-weight containers to be displaced against one another, whereby they can become stuck together. Some containers may be displaced to such an extent that they fall over, whereby jams can occur during conveyance.
Furthermore, it is desirable that all of the containers be subjected to essentially the same treatment in the oven, in order that the foaming action be uniform from one container to another. However, the heating chamber of a typical oven does not establish uniform conditions. That is, in one region of the heating chamber the temperature and/or air current may be different from those of another region. If the containers are being fed through the oven along separate paths (e.g., containers seated on respective opposite sides of the conveyor belt will travel along paths that are laterally spaced from one another), the foaming action may not occur uniformly from one container to another. In an attempt to alleviate that problem, multi-zone ovens have been designed which establish zones of mutually different treatment characteristics, e.g., the temperature, direction of heat flow, and/or direction of air current may intentionally be varied from one zone to another, in an effort to make the overall treatment more uniform among the containers. However, such multi-zone ovens may not adequately achieve that goal and may be more expensive and/or difficult to construct.